Ultrasound lock-in thermography - a defect-selective NDT method for the inspection of aerospace components

Photothermal radiometry and its multiplex version, lock-in thermography have been used for several years for remote non-destructive testing. They are based on propagation and reflection of thermal waves tr which are launched from? the surface into the inspected component by absorption of modulated radiation. Phase angle images obtained by superposition of the initial thermal wave and its reflection display hidden structures down to a certain depth underneath the surface. Elastic waves sent into the component propagate inside the sample until they are converted into heat. A defect causes locally enhanced losses and consequently selective heating lip. Therefore, amplitude modulation of the injected elastic wave turns a defect into a thermal wave transmitter whose signal is detected at the surface by lock-in thermography?; synchronised to the frequency of amplitude modulation. In this way. ultrasound lock-in thermography (ULT) allows for selective defect detection which enhances the probability? of defect detection in the presence of complicated intact structures. Measurements were performed on various kinds of typical defects ill aerospace structures (both metal anti non-metal). The obtained phase angle images reveal areas of hidden corrosion, cracks in rows of rivets, disbonds. impacts, and delaminations. In all these cases the intact structure was suppressed since it heats lip much less in the elastic,wave field. We present examples tl which are relevant, for example for maintenance and inspection of aircraft.